In a conventional air conditioner used in hotels or the like, a room temperature setting means having a dial with a temperature scale is provided in each room so that the room can be kept at a temperature desired by a guest. For example, if a guest sets the dial at 24.degree. C., the temperature in the room is maintained at 24.degree. C. For that purpose, the conventional air conditioner is generally provided with a plurality of switch contacts which are corresponding respectively to a plurality of temperature values predetermined at equal intervals, and each of which is applied with a voltage when the room temperature reaches its corresponding value, and also provided with means for maintaining the room temperature at a desired value by connecting a common contact to the switch contact corresponding to the set temperature value upon setting of that value.
However, if a guest leaves his room with the room temperature set at 24.degree. C., for example, such a conventional unit will continue to keep the temperature in the vacant room at 24.degree. C., causing waste of energy.